Swing-type excavation machines, for example hydraulic excavators and front shovels, require significant hydraulic pressures and flows to transfer material from a dig location to a dump location. These machines direct high-pressure fluid from an engine-driven pump of a swing circuit through a swing motor to accelerate a loaded work tool at the start of each swing, and then restrict the flow of fluid exiting the swing motor at the end of each swing to slow and stop the work tool. Such machines also generally include a boom circuit with its own pump and actuators that together function to raise and lower the work tool simultaneous with or independent of the swinging motion.
One problem associated with this type of hydraulic arrangement involves efficiency. In particular, the fluid exiting the swing motor at the end of each swing and/or exiting the lift actuators during lowering of the work tool can be under a relatively high pressure due to the momentum and/or weight of the loaded work tool. Unless recovered, energy associated with the high-pressure fluid may be wasted. In addition, restriction of this high-pressure fluid during swing deceleration can result in heating of the fluid, which must be accommodated with an increased cooling capacity of the machine.
One attempt to improve the efficiency of a swing-type machine is disclosed in U.S. Pat. No. 7,905,088 of Stephenson et al. that issued on Mar. 15, 2011 (the '088 patent). In particular, the '088 patent discloses a energy recovery system having a boom circuit tied to a swing circuit by way of a shuttle valve, a pressure-actuated valve, and a control valve. During times when the swing rotation of the machine is coming to a stop, pressurized fluid from the swing circuit pushes through the shuttle valve and pressure-actuated valve to enter an accumulator of the boom circuit. This stored fluid can then either be used by the boom circuit to assist movement of boom cylinders, or alternatively directed back through a swing motor of the swing circuit via the control valve in place of or to supplement fluid flow from a swing pump.
Although the energy recovery system of the '088 patent may help to improve efficiencies of a swing-type machine in some situations through storage and re-use of pressurized fluid, it may still be less than optimal. In particular, there may be times when excess stored fluid exists, but neither the boom or swing circuits are in need of the fluid or the pressure of the excess fluid is unsuitable for capture or reuse. During this time, no additional energy capture can be accomplished and the associated energy may be wasted. In addition, the single accumulator of the '088 patent may need to be very large in order to capture all the available fluid from both circuits. The size of such an accumulator may make packaging difficult and increase a size and cost of the associated machine. Further, the number of operations available for re-using the captured energy may be limited in the energy recovery system of the '088 patent.
The disclosed energy recovery system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.